MIMO (Multi-Input Multi-Output) is a technology in which different data sequences are transmitted from a plurality of antennas, respectively and in a reception side, separation of the data sequences that are simultaneously transmitted from each antenna is made possible. By this technology, a transmission rate can be improved without expanding a frequency band. MIMO is applied to for example, IEEE 802.11n standard for wireless LAN (Local Area Network), UMTS LTE standard being developed by 3GPP that is a standard for a next generation mobile communication, or mobile WiMAX standard based on IEEE 802.16e standard. In the above description, IEEE is an abbreviation of “Institute of Electrical and Electronic Engineers”. 3GPP is an abbreviation of “3rd Generation Partnership Project”. UMTS is an abbreviation of “Universal Mobile Telecommunication System”. LTE is an abbreviation of “Long Term Evolution”. WiMAX is an abbreviation of “Worldwide Interoperability for Microwave Access”.
FIG. 3 is a figure for explaining the operating principle of MIMO. Two antennas T-ANT1 and T-ANT2 simultaneously transmit a data sequence A and a data sequence B, respectively. Two antennas R-ANT1 and R-ANT2 receive different sequences in which the data sequence A and the data sequence B are multiplexed, respectively. The data sequence A and the data sequence B from the two antennas T-ANT1 and T-ANT2 are separated from each other by estimating four channel variations h11(t), h12(t), h21(t), and h22(t) at the reception side. In order to make possible to uniquely identify the data sequence from each antenna and estimate the channel variations h11(t), h12(t), h21(t), and h22(t), a pilot signal is embedded in the transmission data sequence of each antenna.
Here, a configuration in which T transmission side antennas and R reception side antennas are included is denoted by T×R. For example, in the specification of LTE, as the configuration of T×R, 2×2, 4×2, and 4×4 are defined for a downlink (data transmission from a base station to a portable terminal). Namely, when the portable terminal of LTE supports MIMO, a plurality of antennas have to be installed in the portable terminal.
On the other hand, patent document 1 describes a wireless system that is composed of a mobile telephone terminal 3, a connection terminal 4 that is detachably attached to the mobile telephone terminal 3, and a portable terminal 5 that is wirelessly connected to the connection terminal 4. In this wireless system, information from the mobile telephone terminal 3 is wirelessly transmitted to the portable terminal 5 via the connection terminal 4 and the information is displayed in the portable terminal 5. On the other hand, information from the connection terminal 5 is received by the connection terminal 4 and inputted to the portable terminal 3, and the portable terminal 3 is controlled.